I've sorted the cases by uniform case thickness and weight. All cases have been trimmed to uniform thickness, just below max length.

Problem:
The case won't chamber 100%. The case slides forward into the chamber until it seems that the shoulder of the case or neck enters the neck / shoulder part of the chambering then there's trouble. This never happens with new unfired cases.

It seems to me that there is a doughnut effect at the junction between the case body and shoulder. This does not happen with every case, probably 1 in 5 cases. Sometimes this doughnut is not visible to the eye. (it is also quite possible the this may not be the problem and that I'm missing something else).

Can anybody shed some light oin what's happening ? Is it possible on seating the bullet that the bullet is pushing down on the neck thus deforming the shoulder ?

I've sorted the cases by uniform case thickness and weight. All cases have been trimmed to uniform thickness, just below max length.

Problem:
The case won't chamber 100%. The case slides forward into the chamber until it seems that the shoulder of the case or neck enters the neck / shoulder part of the chambering then there's trouble. This never happens with new unfired cases.

It seems to me that there is a doughnut effect at the junction between the case body and shoulder. This does not happen with every case, probably 1 in 5 cases. Sometimes this doughnut is not visible to the eye. (it is also quite possible the this may not be the problem and that I'm missing something else).

Can anybody shed some light oin what's happening ? Is it possible on seating the bullet that the bullet is pushing down on the neck thus deforming the shoulder ?

Probably your problem, unless your sizing die is dirty and doing the same thing. Is it a new die? Did you clean it? What kind of lube are you using? Do they ALL chamber before seating a bullet? Do you have a sizing ball on the decapping pin?

AJ

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thanks for the replies. The "doughnut" is between the shoulder and case body on the outside (from what I have seen) and not in the traditional sense when neck trimming. Will post a pic later. (have not cut the necks)

the setup of the dies is a good point. I have recently got a gauge to measure the set back of the shoulder but have not yet loaded ammo with the gauge. I had previously setup the dies by screwing in the body to the shell plate and then a touch further in.

Will check if the die is dirty but that wouldn't expalin that only 1 in about every 5 is giving a problem. using Imperial sizing wax (?) and do have the sizing ball in.

If your seating die is adjusted too deep into the press (bottom of the die bumps the shell holder or is too close to it), you may be accidently crimping the case mouth just a bit. If so, this "can" cause a slight bulge outward of the case right at the point where the neck begins to taper outward to become the shoulder.

the setup of the dies is a good point. I have recently got a gauge to measure the set back of the shoulder but have not yet loaded ammo with the gauge. I had previously setup the dies by screwing in the body to the shell plate and then a touch further in.

My guess is you are sizing the shoulder down too far and creating a small microscopic ridge that you are seeing as the donut. Had this happen to me once. Mftr directions are designed to be lawyer proof and not conducive the max accuracy.

Back the die off 1/8 inch and start back down sizing until they will just chamber. Also use a bump guage to measure, virgin, once fired and 3x fired. Normally by 3x, the brass has flowed to chamber dimensions.